[Haskell-beginners] numerical types, the $ operator

[Antoine Latter]

2009/3/29 Zachary Turner <divisortheory at gmail.com>:
>
> The "pipelining" operator I defined should definitely be used with care.
> For example, it requires the pipelined argument to be the last argument,
> which is not always the case.  And I've also found that with it I tend to
> think about the problem less, and write less efficient code as a result.
> For example given a list of integers, an easy and very readable way to
> remove all multiples of 2 from a list, and then double the remaining items
> could be like this:
>
> let doit x = x |> filter (\y -> y `mod` 2 == 0) |> map (* 2)
>
> as opposed to the more efficient
>
> doit [] = []
> doit (x:xs) | (x `mod` 2 == 0) = doit xs
> doit (x:xs) = (2 * x) : doit xs
>
> since the list is walked twice.  (I'm betting someone will respond with a
> cool one-liner here involving function composition or something else that I
> can't think of yet :)
>

Have you tried timing it?  I'm not seeing much difference in execution
speed between the two functions.

Antoine